The instant invention is concerned with improvements to a servo-mechanism of the type which may be used with a disk drive to cause the read/write head to be moved from track to track on the disk installed in the disk drive. This operation is usually called a "head seek" in the parlance of the disk drive industry since the read/write head is caused to seek a new track in response to a "seek command". The disk installed in the disk drive can either be a permanently installed disk (in which case the disk is usually a "hard disk") or a removable disk (in which case the disk is most often a "floppy disk" although removable hard disks are known.)
The data recorded on the disks are typically recorded in concentric tracks and the read/write head is caused to move between tracks by the aforementioned seek command. The data are stored in a media which may be coated on a disk substrate and the media usually stores the data magnetically, although optical means for data stored are also well known.
In the prior art, both stepper motors and D.C. motors controlled by servo systems have been used to move the read/write head from track to track. Stepper motors are used in the prior art since they are relatively inexpensive and easy to control. D.C. motors and servo systems have been used in the prior art, although they tend to be more complicated than stepper motors systems, since they have faster seek times. That is, the performance of the servo systems is higher than the performance of stepper motor systems and, not surprisingly, the cost of servo motor systems tends to be higher than the cost of stepper motor systems.
In the prior art, disk drives have normally been operated on a flat, horizontal surface, such as a desk top. Sometimes the drive itself has been operated vertically, but more often it has been operated horizontally. In any event, the carriage on which the read/write head is disposed has been arranged horizontal to the horizontal surface of the desk so that the servo system does not have to operate against the force of gravity. Recently, there have been introduced portable personal computer systems which can either operate horizontally or which can be operated at an angle to the horizontal. Typically, such portable personal computer systems have feet disposed along the front edge thereof which may be selectively pulled down such as to rotate the portable personal computer system ten to fifteen degrees with respect to the horizontal. Such portable personal computer systems typically have disk drives and a monitor built into them and, if the monitor is angled upwardly, it makes for easier viewing by the user of the portable personal computer.
Apparently, due to the small size of these personal portable computers, it is preferable to rotate the entire portable personal computer, including built-in monitor, as opposed to trying to angle the monitor with respect to the case which holds the computer and monitor. Rotating the personal computer means that its installed disk drive is also rotated. If the disk drive is a high performance disk drive, it likely has a servo control system, and if rotating the disk drive causes the servo control system to operate either with or against gravity (depending on which way the read/write head is moved), the performance of the servo control system is likely to degrade, or worse, the drive will fail.
Also, in the prior art servo control systems, the read/write head is acted upon by a flex cable having a spring constant which is assumed to be equal to zero. In practice, the spring constant is not ideal spring and therefore its spring "constant" is, in actuality, a variable which varies (from zero) as a function of track position. However, since prior art servo systems have assumed this variable to be a constant, the performance of such systems is less than it could be.
As a result, there is a need to be able to operate a disk drive not only at the horizontal, but at various angles to the horizontal. While disk drives which use stepper motors may have little difficulty in operating at the horizontal and at various angles thereto, disk drives which use servo-mechanisms to control the position of their read/write heads operate very well in the normal horizontal position, but when rotated to the horizontal, their performance tends to degrade and at high enough angles they can become inoperable. As the performance of personal computers improves, it is desirable to equip them with high performance disk drives, such as disk drives which use a servo-mechanism to control the read/write head thereof. As a result, there is a need for a head positioning servo-mechanism which can be used in a drive which may be either used at the horizontal or at an angle thereto.
There is also a need for an improved servo control system which can compensate for the fact that the flex cable is not an ideal spring, but rather has a spring "constant" which varies from zero.